Corn's Buried Secrets

Evaluating a corn crop is kind of like finding a mate: What you see at first glance is interesting, but it doesn’t tell you much. If you don’t learn more, you might make a serious mistake. Whether it is corn or people, what matters most is what lies beneath the surface.

That’s why the most important tools that Farm Journal Field Agronomist Ken Ferrie and Associate Field Agronomist Missy Bauer use in the corn field are backhoes, shovels, soil probes, hatchets and knives. "You’ve got to look belowground," Ferrie says.

What you’re looking for. "If you don’t know what’s going on with your corn crop’s roots, you are likely to blame problems incorrectly on products such as seed, fertilizer or herbicides," Bauer says. She and Ferrie dished the real dirt about evaluating roots, soil condition and the effects of tillage at last summer’s Farm Journal Corn College.

"When we look beneath the surface, we’re trying to identify soil density changes," Bauer explains. "You can think of density as the amount of air space between soil particles—the less air space, the more dense the soil is.

"It’s like a 5-gal. bucket full of golf balls compared to a 5-gal. bucket full of peas. There’s far more air space among golf balls than among peas," she says. "With corn, we are concerned with what happens to roots when they encounter abrupt changes in soil density. Roots grow horizontally along dense layers, rather than downward at a 30° to 40° angle.

"If roots grow along a horizontal layer, they may not penetrate to depths where they can reach water and nutrients," Bauer adds. "Things always look a lot better on top if there is uniform soil density down below."

Dense layers also keep water from infiltrating. "In the spring, density layers can hold water up, causing the soil to stay saturated for a longer period of time or to pond on the surface," she says.
"If it stays wet too long, you may sacrifice yield by planting late or in less than ideal conditions. Saturated soils also can lead to seedling diseases and denitrification."

Roots can tell you how your crop is faring—but it takes the tools we mentioned to give them a voice.

In this soil pit, the blue marker on the left identifies a shallow tillage layer put in by a soil finisher. The density change can also be seen to the right of the middle corn plant, where loose soil created by the soil finisher has been swept away. The blue marker on the right marks a tillage layer, or density change, where a moldboard plow went through. No residue has been buried beneath this layer, but a buried cornstalk is visible above and to the right of the marker.

Start with a soil pit. "If you have never dug a soil pit, I strongly recommend you do it," Bauer says.

"A soil pit adds to your understanding because you can see everything that’s going on," Ferrie says. "Of course, you can’t dig a pit in every field. But digging one or two pits will give you knowledge that will help you interpret what you discover with your spade and tile probe."

In the pit, Bauer performs a three-step inspection: feeling the condition of the soil with the aid of a knife; evaluating root growth; and looking for moisture layers.

"Work the soil with a knife, from the top down," Bauer says. "You will soon learn to feel changes in soil density, where it’s looser or tighter. Where there is a change in density, measure the depth from the surface. See if the change in density, or the new layer, is turning the plants’ roots."

Density changes are typically created by horizontal tillage tools, such as a disk, field cultivator or moldboard plow. In some soil pits, you may find more than one layer.

For example, a moldboard plow will put in one density layer. Then, when a field cultivator or soil finisher runs in the spring, it will put in another. After planting, corn roots will penetrate the loose, fluffy, field-cultivated soil, then turn horizontally when they reach the denser soil underneath.

The first, second and third sets of crown roots are very important for yield because they take up a lot of water and nutrients. If those roots are being turned by dense soil layers, yield potential can be reduced.

At this point, look at root growth. See if the crown roots are reaching down to the subsoil.
Dense layers can also affect moisture movement. "You may see moisture deep in the soil, even though the top few inches are dry, because roots could not penetrate the dense layer and pull the water up," Bauer explains. "Your goal is to have uniform soil moisture in the profile, from top to bottom."

Dig roots. After you study your soil pit, take a spade, dig up some plants at various areas in the field and study the roots.

"Select rows in which there are no wheel tracks," Bauer says. "When you dig up a plant, keep your spade outside the area where the planter gauge wheels ran—4" or 5". Dig about 10" deep, so you get through all the soil layers that are present.

"As you dig out the root ball, watch for soil to break off naturally where there is a change in density," she says.

"Use your hatchet to chop off the top of the plant. Shake off loose soil from the roots. You may see roots are growing sideways instead of downward." With your hatchet, cut off the first round of brace roots. Tap the root ball a bit to loosen more soil, then chop off the second set of brace roots. Be careful not to chop off the crown roots. Clean off the rest of the soil.

The roots of this plant began growing downward, but turned after encountering a dense layer of soil.

Observe the roots. "Are any roots turned horizontally? If so, at what depth? That tells you the depth of the dense layer," Bauer says.

"If roots are turned, check whether it is the first, second or third set of crown roots. Those roots are very important for yield because they take up a lot of water and nutrients. If a lot of them are turning, it’s affecting the yield potential of those plants."

To identify the crown roots, find the old seed. "The old seed is attached to the mesocotyl," Bauer says. "Follow the mesocotyl up to the base of the crown. The crown roots also attach to the crown, emerging in sets of three or four, at different growth stages."

Probe for yield. The last step is probing to locate changes in soil density.

"Make sure you’re not in a wheel track or where the gauge wheel ran," Bauer says. "Push the probe down, feeling for resistance, where it’s harder to push through. Record the depth of the first layer you find. Then go down again, push through that layer, feel for the next layer and record its depth."

A penetrometer makes comparisons easier because it has a psi scale to help you find the layers. "You don’t really need to assign numbers because you are simply comparing one area with another," Ferrie says.

Where a vertical tillage tool was set too shallow, columns of unworked soil are visible between tilled areas. Although it may be concealed by the effect of a vertical-till harrow, planter units will bounce or vibrate as they encounter tilled and untilled soil. If you see this, consider using a horizontal tillage tool, such as a field cultivator, to create a uniform seedbed.

When operated at the correct depth, a vertical tillage tool shatters soil uniformly, all the way to the surface and across the width of the implement.